Unlike conventional random access memory (RAM) chip technologies, in magnetic RAM (MRAM), data is stored by magnetization of storage elements. The basic structure of the storage elements consists of metallic ferromagnetic layers separated by a thin tunneling barrier. One of the ferromagnetic layers (e.g., the ferromagnetic layer underneath the barrier) has a magnetization that is fixed in a particular direction, and is commonly referred to as the reference layer, which is interchangeably referred to as the fixed layer. The other ferromagnetic layers (e.g., the ferromagnetic layer above the tunneling barrier) have a magnetization direction that may be altered to represent either a “1” or a “0,” and are commonly referred to as the free layers.
For example, a “1” may be represented when the free layer magnetization is anti-parallel to the fixed layer magnetization. In addition, a “0” may be represented when the free layer magnetization is parallel to the fixed (reference) layer magnetization or vice versa. One such device having a fixed (reference) layer, a tunneling layer, and a free layer is a magnetic tunnel junction (MTJ). The electrical resistance of an MTJ depends on whether the free layer magnetization and fixed layer magnetization are parallel or anti-parallel to each other. A memory device such as MRAM is built from an array of individually addressable MTJs.
To write data in a conventional MRAM, a write current, which exceeds a critical switching current, is applied through an MTJ. Application of a write current that exceeds the critical switching current changes the magnetization direction of the free layer. When the write current flows in a first direction (from a free layer to a reference layer), the MTJ may be placed into or remain in a state in which its free layer magnetization direction and reference layer magnetization direction are aligned in a parallel orientation. When the write current flows in a second direction (from the reference layer to the free layer), opposite to the first direction, the MTJ may be placed into or remain in a second state in which its free layer magnetization and fixed layer magnetization are in an anti-parallel orientation.
To read data in a conventional MRAM, a read current may flow through the MTJ via the same current path used to write data in the MTJ. If the magnetizations of the MTJ's free layer and fixed layer are oriented parallel to each other, the MTJ presents a parallel resistance. The parallel resistance is different than a resistance (anti-parallel) the MTJ would present if the magnetizations of the free layer and the fixed layer were in an anti-parallel orientation. In a conventional MRAM, two distinct states are defined by these two different resistances of an MTJ in a bitcell of the MRAM. The two different resistances indicate whether a logic “0” or a logic “1” value is stored by the MTJ.
Spin-transfer torque magnetic random access memory (STT-MRAM) is an emerging nonvolatile memory that has advantages of non-volatility. In particular, STT-MRAM embedded with logic circuits may operate at a comparable or higher speed than off chip dynamic random access memory (DRAM). In addition, STT-MRAM has a smaller chip size than embedded static random access memory (eSRAM), virtually unlimited read/write endurance as compared with FLASH, and a low array leakage current.
In particular, spin-transfer torque (STT) efficiency and retention are specified parameters in the design of the MTJ for an embedded STT-MRAM. As a result, perpendicular STT-MRAM has become a leading candidate for providing next-generation embedded non-volatile memory. While STT-MRAM is a promising candidate for use as a unified memory for a low power MCU (memory control unit) or IoT (Internet of things) applications, STT-MRAM is still not fast/low-power enough to serve as cache replacement memory (e.g., low level cache (LLC) or otherwise).